Lamp envelope with a thin transparent buffer film on its inner surface

ABSTRACT

The inner surface of a soda-lime-silicate glass envelope for a fluorescent lamp or similar mercury-discharge device is coated with a thin film of submicroscopic fibrous crystals of boehmite (A10(OH)) prior to the phosphor coating operation. The envelope is subsequently heated to thermally decompose the boehmite crystals in situ and transform them into fibrils and rodlike particles of gamma alumina (Al2O3) that are bonded to the glass surface. The heating is advantageously achieved during the bulblehring operation after the bulb is coated with phosphor. The gamma alumina particles chemically react with the sodium and other alkali constituents of the glass to form inert compounds and thus provide a transparent &#39;&#39;&#39;&#39;buffer&#39;&#39;&#39;&#39; film which, in the finished lamp, inhibits the formation of black alkali-mercury deposits on the inner surface of the envelope during lamp operation. The lumen maintenance of the lamp is accordingly enhanced. Deposition of the boehmite crystals is achieved by coating the envelope interior with a 0.5 percent to 5 percent aqueous solution of a colloidal boehmite complex that consists of 85 percent by weight crystalline boehmite, having 13 percent by weight of acetic acid and 2 percent by weight of water attached to the boehmite fibrils. Transformation of the boehmite into rodshaped gamma alumina is achieved by heating the envelope to a temperature of at least 400* C. percent by weight of acetic acid and 2 percent by weight of water attached to the boehmite fibrils. Transformation of the boehmite into rod-shaped gamma alumina is achieved by heating the envelope to a temperature of at least 400* C.

United States Patent [72] Inventor Rudolph Nagy Lake Elsie, Wis. [21]Appl. No. 22,478

[22] Filed Mar. 25, 1970 Division of Ser. No. 776,624, Nov. 18,1968,Pat. No. 3,541,377.

[45] Patented Nov. 2, 1971 [73] Assignee Westinghouse ElectricCorporation Pittsburgh, Pa.

[54] LAMP ENVELOPE WITH A THIN TRANSPARENT BUFFER FILM N ITS INNERSURFACE 6 Claims, Drawing Figs.

[56] References Cited UNITED STATES PATENTS 3,094,641 6/1963 Gungle etal 313/109 3,424,606 l/l969 Giudici 117/97 X Primary Examiner-Alfred L.Leavitt Assistant Examiner-Edward G. Whitby Attorneys-A. T. Stratton, W.D. Palmer and D. S. Buleza ABSTRACT: The inner surface of asoda-lime-silicate glass envelope for a fluorescent lamp or similarmercury-discharge device is coated with a thin film of submicroscopicfibrous crystals of boehmite (A(OH)) prior to the phosphor coatingoperation. The envelope is subsequently heated to thermally decomposethe boehmite crystals in situ and transform them into fibrils androdlike particles of gamma alumina (A1 0 that are bonded to the glasssurface. The heating is advantageously achieved during the bulb-lehringoperation after the bulb is coated with phosphor. The gamma aluminaparticles chemically react with the sodium and other alkali constituentsof the glass to form inert compounds and thus provide a transparentbuffer" film which, in the finished lamp, inhibits the formation ofblack alkali-mercury deposits on the inner surface of the envelopeduring lamp operation. The lumen maintenance of the lamp is accordinglyenhanced.

Deposition of the boehmite crystals is achieved by coating the envelopeinterior with a 0.5 percent to 5 percent aqueous solution of a colloidalboehmite complex that consists of percent by weight crystallineboehmite, having 13 percent by weight of acetic acid and 2 percent byweight of water attached to the boehmite fibrils. Transformation of theboehmite into rod-shaped gamma alumina is achieved by heating theenvelope to a temperature of at least 400 C. percent by weight of aceticacid and 2 percent by weight of water attached to the boehmite fibrils.Transformation of the boehmite into rod-shaped gamma alumina is achievedby heating the envelope to a temperature of at least 400 C.

HEAT ENVELOPE TO APPROX. C TO DRY FLUSH-COAT ENVELOPE WITH AQUEOUSSOLUTION OF COLLOIDAL ALUMINA HEAT ENVELOPE TO DRY COATING FLUSHENVELOPE WITH PHOSPHOR LACOUER, DRY AND LEHR LAMP ENVELOPE WITII A TIIINTRANSPARENT BUFFER FILM ON ITS INNER SURFACE CROSS-REFERENCE TO RELATEDAPPLICATION This application is a division of pending application Ser.No. 776,624 filed Nov. 18, l968, now Pat. No. 3,541,377.

BACKGROUND OF THE INVENTION 1 Field of the Invention This inventionrelates to electric lamps and has particular reference toan improvedvitreous envelope for a fluorescent lamp or similar device thatgenerates light by means of a lowpressure mercury-vapor discharge.

2 Description of the Prior Art As is well known, the light output offluorescent lamps gradually decreases as the lamps are burned. lthasbeenfound that one of the contributing factors to this progressive loss oflight output is the discoloration of the inner surface of the envelopeproduced by the reaction of mercury with sodium that is present withinthe soda-lime-silicate glass from which the envelope is fabricated. Thesodium apparently diffuses outof the glass to the inner surface of theenvelope during the lehringoperation when the envelope is heated to atemperature of around 600 C, in order to remove the binder from thephosphor coating. As the finished lamp is burned, mercury ions, from thedischarge combine with the sodium and (possibly other alkali ions) onthe inner surface of the envelope and form a mercury-alkali amalgam thatis brownblack in color and thus reduces the amount of light transmittedby the envelope.

In order to prevent theformation of such mercury-alkali amalgam depositsand the resultant loss of light output, barrier layers of finely dividedrefractory oxides such as Al,0, Sit), and Ti in transparent thicknesseshave heretofore been applied to the inner surface of the glass envelope.Such barrier layers are formed by suspending the refractory oxideparticles in an organic vehicle, such as nitrocellulose orethylcellulose, to form a lacquer which is coated onto the inner surfaceof the bulb and dried. The glass envelopes are then baked or lehred at atemperature just below the temperature at which the glass envelopedeforms (550 C. to approximately 600 C. for sodalime-silicate glass) tovaporize the organic vehicle and affix a protective layer of refractoryoxide particles to the glass. The inner surface of the bulb was thenphosphor coated and lehred in the usual fashion. The barrier layer wasthus interposed between the phosphor coating and the inner surface ofthe envelope and prevented the mercury ions from contacting andcombining with sodium that may have diffused to the inner surface of thebulb during the high-temperature lchring operations.

A fluorescent lamp having a barrier layer of the aforesaid type isdisclosed in U.S. Pat. No. 3,067,356, issued Dec. 4, I962 to J. G. Ray.A more recent proposal involves the use of a thinner barrier layer oftitanium dioxide or zirconium dioxide that contains an additionalmaterial such as magnesium oxide, barium oxide, lead oxide or zincoxide. The titanium or zirconium oxide is applied to the envelope intheform of a metallic-organic compound which is then converted to theoxide of the respective metal. A fluorescent lamp having such a modifiedbarrier layer is disclosed in U.S. Pat. No. 3,3 77,494, issued Apr. 9,1968 to R. W. Repsher.

While the aforementioned barrier layers achieve the desired result ofphysically shielding the alkali-containing inner surface of the bulbfrom the mercury ions in the discharge, they require an organic bindersuch as nitroc llulose or ethyl-cellulose, or a plurality of refractoryoxides and metallic-organic compounds, thus complicating the lampmanufacturing operations and increasing the cost of the lamps.

SUMMARY OF THE INVENTION It is accordingly the general object of thepresent invention to provide a simple and inexpensive means forpreventing the envelope of a fluorescent lamp or similar mercurydischarge device from becoming discolored as the lamp is burned.

A more specific object is the provision of a lamp envelope that containsan alkali-metal oxide constituent, such as Na,0 or K 0, and can becoated with phosphor and subjected to the other lamp-making operationswithout causing such constituents to subsequently form ,discoloringdeposits within the finished lamp in the presence of mercury ions.

The foregoing objects of the invention and other advantages which willbecome apparent are achieved by forming an integral film of materialon'the inner surface of the lamp envelope that provides a bufferingaction at the phosphor-glass interface. More specifically, a thintransparent film of rodshaped gamma alumina (A1 0 particles if formed onthe inner surface of the glass envelope which chemically reacts with thealkali constituents of the glass, such as sodium or potassium, thatdiffuse to the inner surface of the envelope during or after lampfabrication and converts such alkalis into sodium aluminate (NaAl0 orpotassium aluminate (KAlO,).

The buffer film of rod-shaped A1 0 particles thus renders the boehmitecrystals, drying the resulting coating, coating the treated bulb withphosphor-containing lacquer in the regularmanner and then baking theenvelope at approximately 600 C. to remove the organic binder from thephosphor and thermally decompose the boehmite crystals and convert theminto rodlike gamma alumina particles that are bonded directly to theglass surface. The colloidal boehmite crystals are. accordingly,converted in situ into rodlike gamma alumina fibrils during the normalsequence of operations required to make the lamp. The thermal conversionof the boehmite crystals into rodlike gamma alumina particles can, ofcourse, also be achieved by heating the envelope before it is coatedwith phosphor and lehred. Satisfactory results have been obtained bytreating the bulbs with a 0.5 to 5 percent aqueous solution of colloidalboehmite and a 2.5 percent solution is preferred. From 0.1 percent to 1percent by weight of barium acetate (Ba(C2H O2) H2O) can 2 be added tothe colloidal solution of boehmite to remove any sulfates that may bepresent on the glass surface by converting them to nonreactive @lQ l ilZ* 3Q l' BRIEF DESCRIPTION OF THE DRAWING A better understanding of theinvention will be obtained by referring to the accompanying drawing,wherein:

FIG. 1 is a front elevational view of a fluorescent lamp having anenvelope that includes a transparent "buffer" film of rod-shaped gammaalumina particles in accordance with the present invention;

FIG. 2 is an enlarged fragmentary cross-sectional view of thephosphor-coated envelope taken along the line ll-ll of FIG. 1;

FIG. 3 is a block diagram illustrating the sequence of operationsfollowed in forming the film of gamma alumina on the inner surface ofthe envelope in accordance with a preferred embodiment of the invention;and,

FIGS. 4 and 5 are photornicrographs illustrating the physicalcharacteristics of the rod-shaped gamma alumina particles formed in situaccording to the present invention and the finely divided aluminaparticles employed in the prior art barrier layers, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT While the present invention canbe used in various types of mercury discharge devices which havevitreous envelopes that contain alkali-oxide constituents, it isespecially adapted for use in conjunction with fluorescent lamps and hasaccordingly been illustrated and will be described in this form.

With specific reference to the drawing, in FIG. 1 there is shown afluorescent lamp 10 having a tubular glass envelope [2 that ishermetically closed at each end by the usual mount assemblies consistingof glass stems 14 that are sealed to the envelope and support suitableelectrodes I that are attached to the respective stems by lead wires 16and 17. The electrodes 15 comprise tungsten wire coils coated withelectron emissive material, as is well known in the art, and the leadwires 16 and 17 are sealed through the glass stems 14 and electricallyconnected to contact pins 18 carried by base members 20 fastened to thesealed ends of the envelope 12. Prior to being sealed, the envelope 12is charged with a filling of suitable inert ionizable starting gas, suchas argon at a pressure of4 millimeters, and a small dose of mercury.

In accordance with the present invention, the inner surface of theenvelope 12 is provided with a thin transparent film 22 of rodlikeparticles of gamma alumina that are bonded to the surface of the glass.As is shown more clearly in FIG. 2, the film 22 ofgamma alumina islocated between the inner surface of the envelope 12 and a layer 24 of asuitable ultravioletresponsive phosphor such as calcium halophosphateactivated by manganese and antimony. In accordance with standardlamp-making practice, the envelope I2 is composed of sodalime-silicateglass that contains up to 16 percent by weight of Na 0 and up to 3percent by weight K 0.

The film 22 of rodlike gamma alumina is formed by preparing an aqueouscolloidal solution of boehmite crystals and flushing the envelopeinterior with this solution and drying it to form a thin film ofboehmite crystals on the glass surface. Boehmite is a mineral found inbauxite and, more specifically, is an orthorhombic form of aluminumoxide and hydroxide A10(OI'I). Boehmite is thus hydrous aluminous oxide.The crystals of boehmite are needle-shaped, submicroscopic in size (0.1micron or less) and fibrous and, when suspended in water, produce acolloidal solution having a positive ionic charge. The pore diameter ofthe boehmite crystals is only 47 A. and, by virtue of their small sizeand the positive charge on the colloid, a very thin coherent andmicroporous film of these crystals is produced on the glass. A colloidalboehmite complex which has these properties, is soluble in water andconsists of 85 percent by weight A10(OH) crystals with 13 percent byweight acetic acid (CH COOH) and 2 percent by weight H 0 attached to thecrystals is commercially available under the trade name Baymal (DuPontde Nemours & Company). Satisfactory results have been obtained by usingan 0.5 to 5 percent solution of the aforementioned colloidal aluminacomplex in distilled water and a 1 percent to 2.5 percent solution ispreferred. Aqueous solutions containing more than 5 percent by weight ofthe aforesaid boehmite complex produces films of too great a thicknessresulting in a glassy or smooth surface to which the phosphor coatingdid not adhere readily.

After the thin film of boehmite crystals has been deposited on the innersurface of the envelope 12 as above described, the envelope is coatedwith a phosphor paint or lacquer consisting ofa suitable vaporizablevehicle, such as ethylcellulose, and suspended phosphor particles. Thephosphor lacquer is then dried and the bulb is lehred or baked at atemperature of about 550 to 650 C. for about one minute to vaporize theethylcellulose binder and thermally decompose the boehmite crystals andconvert them in situ into rodlike particles of gamma alumina. Thismaterial has an area of 300 to 350 square meters per gram and thus formsa very thin continuous film on the inner surface of the envelope 12. Thethickness of the film 22 ofgamma alumina does not exceed about 1 micron(10,000 A. units) and coatings much thinner than this can be readilyformed by reducing the concentration of the colloidal boehmite crystalsin the aqueous solution. It is important to note that such thin filmsare possible as a practical matter in production by virtue of the factthat the fibrous boehmite crystals are decomposed in situ during thebulb-lehring operation and that no organic vehicles or binders orseparate lehring operations are required to accomplish this. It shouldalso be noted that since the boehmite is transformed in situ by heatinto the rod-shaped gamma alumina particles, the latter are bondeddirectly to the glass surface and comprise an integral part of theenvelope 12.

It is believed that during the thermal transformation of the A10(OI-l)into gamma alumina some of the A10(OH) chemically reacts with Na thathas diffused to the inner surface of the envelope l2, and other alkaliconstituents such as K which may be there present, to form NaAlO andother compounds which prevents the alkali ions from reacting withmercury ions and producing black alkali-mercury amalgams. The film ofcolloidal A10(OH) and resulting film of gamma alumina thus act asbuffers on the inner surface of the envelope 12 which prevent theformation of discoloring deposits within the finished lamp.

In FIG. 3 there is illustrated a specific example of the various stepsinvolved in treating a lamp envelope to form a thin tenacious film ofgamma alumina on its inner surface in accordance with the invention. Asshown, the envelope is first washed to remove surface dirt and othercontaminates. A 1

percent solution of hydrofluoric acid can be used for this purpose. Thewashed envelope is then dried by heating it to approximately l50 C. Thiscan be achieved by passing a stream of heated air through the envelope.The dried envelope is then flush coated with the aqueous colloidalsolution of the boehmite complex, and the envelope is again heated toapproximately 150 C. to dry the coating and form a thin film of boehmitecrystals on the inner surface of the envelope. The envelope is thenflushed with the phosphor lacquer, the resulting layer of binder andphosphor particles is dried and the bulb is then lehred at approximately650 C. for one minute to vaporize the organic binder and transform theboehmite crystals into the rod-shaped gamma alumina particles.

As shown in the photomicrograph which constitutes FIG. 4 (magnification40,000 X), the discrete rod-shaped particles 26 of gamma alumina formedon the inner surface of the envelope range from 2,000 A. to about 7,000A. in length and are approximately 500 A. in width. In contrast, thephotomicrograph (magnification 50,000 X and reproduced as FIG. 5) of aprior art barrier layer made in accordance with the teachings of theaforementioned Ray Patent US. Pat. No 3,067,356 shows that the finelydivided particles 28 of prefired A1 0 are regularly shaped particles(mostly hexagonal) and about 500 A. in diameter. The barrier layer ofprefired alumina also had a milky powdery appearance and the finegranules ofAl 0 could very easily be removed simply by rubbing thecoating.

As indicated in table I below, fluorescent lamps having envelopesprovided with in situ formed films of gamma aluminum in accordance withthe present invention have a higher light output compared toconventional lamps of the same type without such coatings.

The data shown in table I was obtained on lamps that were treated with a2.5 percent aqueous solution of Baymal." As will be noted, the use ofthe thin film ofdiscrete particles ofinsitu formed gamma aluminaincreased the lamp efficiency by 2.1 percent after hours burning and 1.2percent after l000 hours of burning.

Since the sodium on the inner surface ofa soda-lime-silicate glassenvelope is normally combined with sulfates to form Na SO 0, it wouldalso be desirable to remove such sulfates from the glass surface andalso the sulfate which results from the oxidization of theethylcellulose binder in the phosphor lacquer which may contain as muchas 0.4 percent sodium sulfate. This is accomplished in accordance withthe present invention by adding a small amount of barium acetate Ba( Q I-I O H O) to the colloidal solution of AlO(OH) so that, upon lehring,nonreactive barium sulfate (BaS0 is formed on the bulb surface. Thiscompound is insoluble and very stable, even under very high energyradiation such as X rays, and has thus been used in intensifying X-rayscreens. Hence, the aforesaid mixture of colloidal A(Ol-l) andBa(CzH1O2)z H2O in a water solution provides an inexpensive practicalmeans for converting sodkum and sulfates which may be present on theinner surface of the lamp envelope into inert compounds that do notimpair lamp performance. As a specific example, from 0.1 percent to 1percent by weight of barium acetate is added to the aqueous colloidalsolution of the boehmite complex (Baymal). Any excess of barium saltthat remains in the film will combine with A1 0 to form BaAlO an inertcompound.

It will be appreciated from the foregoing that the objects of theinvention have been achieved in that a very simple means for chemicallystabilizing the inner surface of a fluorescent lamp envelope has beenprovided which prevents diffused sodium and other alkali constituents inthe glass from combining with mercury ions and forming amalgam depositswhich impair the light output and efficiency of the finished lamp. Theuse of an aqueous colloidal solution of boehmite to deposit a thin filmof boehmite crystals on the inner surface of the bulb which issubsequently transformed in situ into an integral film of rodlike gammaalumina particles eliminates the. organic vehicle and costlyorgano-metallic materials required to form the prior art barrier layers.

ing from the spirit and scope of the invention.

I claim as my invention:

1. A fluorescent lamp envelope that is composed of a glass whichcontains an alkali constituent and has a thin transparent film ofrodlike gamma alumina particles bonded to its inner surface.

2. The fluorescent lamp envelope of claim 1 wherein said rodlikeparticles of gamma alumina are from about 2,000 A to about 7,000 A inlength and approximately 500 A in width.

3. The fluorescent lamp envelope of claim 1 wherein; said glass is asoda-lime-silicate type glass, and said film of rodlike gamma aluminaparticles has a thickness of up to approximately 10,000 A.

4. The fluorescent lamp envelope of claim 1 wherein said film of gammaalumina particles includes BaS0 5. A soda-lime-silicate glass envelopeadapted for use in a fluorescent lamp or similar device, said envelopehaving a film of fibrous boehmite crystals deposited on and bondeddirectly to the inner surface thereof.

6. The lamp envelope of claim 5 wherein said film of fibrous boehmitecrystals is microporous and extends over the entire inner surface of theenvelope.

While one embodiment of the invention has been illustrated anddescribed, it will be appreciated that various modifications in thetypes and quantities of materials used and in the method of treating theenvelopes can be made without departt i t

2. The fluorescent lamp envelope of claim 1 wherein said rodlikeparticles of gamma alumina are from about 2,000 A to about 7,000 A inlength and approximately 500 A in width.
 3. The fluorescent lampenvelope of claim 1 wherein; said glass is a soda-lime-silicate typeglass, and said film of rodlike gamma alumina particles has a thicknessof up to approximately 10,000 A.
 4. The fluorescent lamp envelope ofclaim 1 wherein said film of gamma alumina particles includes BaS04. 5.A soda-lime-silicate glass envelope adapted for use in a fluorescentlamp or similar device, said envelope having a film of fibrous boehmitecrystals deposited on and bonded directly to the inner surface thereof.6. The lamp envelope of claim 5 wherein said film of fibrous boehmitecrystals is microporous and extends over the entire inner surface of theenvelope.